Pulsed thyristor trigger control circuit

ABSTRACT

A trigger control circuit is provided for producing firing pulses for the thyristor of thyristor control system such as a power factor controller. The control circuit overcomes thyristor triggering problems involved with the current lag associated with controlling inductive loads and utilizes a phase difference signal, already present in the power factor controller, in deriving a signal for inhibiting generation of a firing pulse until no load current is flowing from the preceding half cycle and thereby ensuring that the thyristor is triggered on during each half cycle.

ORIGIN OF THE INVENTION

This invention described herein was made by an employee of the UnitedStates Government and may be manufactured and used by or for theGovernment of the United States for governmental purposes and withoutthe payment of any royalties thereon or therefor.

TECHNICAL FIELD

The present invention relates to switching circuitry for thyristors usedwith inductive loads such as a motor, and in particular, used in powerfactor controllers for such loads.

BACKGROUND ART

In U.S. Pat. Nos. 4,052,648 (Nola) and 4,266,177 (Nola), there aredisclosed power factor controllers which are particularly useful inconnection with inductive loads such as induction motors. Thesecontrollers, which sample the line voltage and current through themotor, include a thyristor which controls the power input to the motorin proportion to the detected phase difference between the sampledvoltage and current, such that less power is provided to the motor inresponse to decreasing motor loading.

As is well understood in the art, a thyristor, i.e., an SCR or triac,will switch on if the gate electrode thereof is supplied with a currentpulse whose duration may typically be of only a few microseconds, andwill remain on until the anode current goes to a zero level. If thethyristor is used to control a sinusoidal current in a resistive load,the trigger pulse can be applied during any portion of the sine wavesince the current will also be precisely in phase with the voltage.However, the current in an inductive load significantly lags the voltageand this can create problems in connection with triggering of thethyristor. More specifically, as is explained in more detailhereinbelow, if the firing or trigger pulse is generated at a time whencurrent is flowing from the preceding half cycle (due to the currentphase lag), the triac will already be turned on at that time. Further,when the current goes to zero and the triac goes off, the triac willremain off for an entire half cycle. Thus the trigger pulse will bewithout effect. The disadvantages of such operation in a thyristorcontrol system are evident.

In a power factor controller of the type discussed above, this problemis avoided by supplying thyristor gate current using a fixed levelsignal rather than a trigger pulse. However, it will be appreciated thatsupplying a fixed level signal results in considerably more powerconsumption than supplying a trigger pulse. Moreover, in certain powerfactor controllers employing triacs, wherein the gate power is deriveddirectly from the line voltage it has been found necessary to use asensitive gate pilot triac to turn on the main power triac.

SUMMARY OF THE INVENTION

In accordance with the invention, a triggering circuit for a thyristoris provided which utilizes a trigger pulse to fire the thyristor andwhich inhibits generation of the trigger pulse if current is flowingfrom the preceding half cycle. The circuit is particularly adapted foruse in power factor controllers such as discussed above because it usesa signal already existing in such a power factor controller to inhibitthe firing pulse. Thus, the trigger circuit of the invention eliminatesthe problems discussed above with reference to pulse firing while alsoretaining the advantages of pulse firing over fixed level firing anddoing away with the need for the use of pilot triacs.

According to a preferred embodiment thereof, a triggering circuit isprovided for a thyristor control system for an alternating current inputto other than a completely resistive load (i.e., a load which results ina phase difference between the load current and voltage waveforms), thetrigger circuit comprising pulse producing means for producing firingpulses for firing the thyristor, means for deriving a control signal forsaid thyristor based on the phase difference between the load currentand voltage, and mean responsive to the control signal for inhibitingproduction of a firing pulse until a time when no load current isflowing from the previous half cycle of the alternating current input.Preferably, the pulse producing means comprises an electronic switchingdevice such as a transistor and the firing pulse inhibiting meanscontrols the "on" time of the transistor. Advantageously, the pulsefiring inhibiting means establishes a reference point for triggering thetriac based on the phase difference signal and inhibits producing of afiring pulse in advance of the reference point. The base of the controltransistor for the thyristor is connected to receive a first signal fromthe inhibiting means and a second signal from a control circuit for thethyristor, and in an advantageous embodiment, the thyristor is connectedso as to be turned on only when the signals are negative. The firingangle of the second signal controls the turn on time of the transistor,and thus the thyristor, so long as this firing angle occurs after thereference angle; however, when the firing angle is in advance of thereference angle, the transistor is not turned on until the referenceangle is reached.

Other features and advantages of the invention will be set forth in, orapparent from, the detailed description of the preferred embodimentswhich follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) to 1(f) are waveforms associated with conventionalthyristors, used in explanation of the problem overcome by theinvention;

FIG. 2 is a schematic circuit diagram, partially in block form, of apower factor controller incorporating the trigger control circuit of theinvention; and

FIGS. 3(a) to 3(m) are waveforms associated with the system of FIG. 2and used in explanation of the operation of the trigger circuit of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIGS. 1(a) to 1(f), these figures show waveformsassociated with conventional thyristor operation and which are helpfulin understanding the problem overcome by the present invention. With athyristor connected to an A.C. input to provide an output current whentriggered "on" or fired, and with input current and voltage waveforms asshown in FIG. 1(a), it will be understood that if the firing pulse forthe thyristor occurs at a time such as indicated in FIG. 1(b), theoutput current will be that shown in FIG. 1(c). Similarly, the firingpulse is advanced in time so as to coincide with the zero crossing ofthe current waveform, as indicated in FIG. 1(d), continuous current willflow, i.e., the current will be the same as shown in FIG. 1(a). If thefiring pulse is advanced further in time, as indicated in FIG. 1(e),such that the firing pulse actually occurred when current was flowing inthe previous half cycle (and the thyristor is already "on"), when thecurrent goes to zero, the thyristor is turned off and, because thefiring pulse has already occurred, the thyristor will remain off for theentire ensuring half cycle, as is indicated in FIG. 1(f), It is thisproblem that the present invention seeks to overcome. However, beforeexploring the invention in more detail, a control system in which theinvention is incorporated will now be considered.

Thus, turning now to FIG. 2, there is shown a preferred embodiment ofthe invention as incorporated in a power factor controller or controlsystem similar to that disclosed in U.S. Pat. No. 4,266,177. The systemshown in FIG. 2 is similar to that described in the patent just referredto and the subject matter of that patent is hereby incorporated byreference.

The system of FIG. 2 includes input terminals 10 and 12 which receivethe input waveform (which is typically 115 volts A.C. and is illustratedin FIG. 3(a), and which are connected to a power supply circuit 14 andacross the series combination of the winding or windings of a motor 16,a thyristor (triac) 18 and a current sensing resistor 20. The inputterminal 10 is also connected to positive and negative voltage squaringcircuits 24 and 22 which produce respective oppositely phased, fullwave, rectangular wave outputs "f" and "g", illustrated in FIGS. 3(f)and 3(g), respectively. A signal voltage is developed across currentsensing resistor 20 which is illustrated in FIG. 3(b) for arepresentative mode of operation of triac 18 and in FIG. 3(c) forcontinuous operation (with triac 18 always on), and which is applied tothe inputs of full wave current squaring wave shapers 26 and 28. Waveshaper 26 is responsive only to positive half cycles of the currentwaveform, and in response to the current waveform shown in FIG. 3(b),produces a rectangular output waveform "h" shown in FIG. 3(h). Waveshaper 28 is responsive only to negative half cycles of the currentwaveform and, in response to the current waveform illustrated in FIG.3(b), produces a rectangular output waveform "i" shown in FIG. 3(i).

The outputs "g" and "f" of voltage squaring wave shapers 22 and 24 areconnected to a negative going pulse detector 30 which produces negativespikes used in triggering a ramp generator 32 connected to the output ofdetector 30. The output of ramp generator 32 is connected to thepositive (non-inverting) input of an operational amplifier 34 whichfunctions as a zero crossing detector. A control signal to be describedbelow is connected to the negative (inverting) input of operationalamplifier 34.

The control signal referred to above is a function of (1) a signal basedon the phase difference between the current and voltage applied to motor16 and (2) a command or reference signal to be described below. Thephase difference signal is derived by a selected combination of theoutputs of shapers 22, 24, 26 and 28. Specifically, the outputs ofshapers 22 and 26 are summed in a summing circuit or summer 36 and theoutputs of shapers 24 and 28 are summed in a summer 38. The signals soproduced are rectified by diodes 40, 42 and summed at summing point 43to provide the output signal "j" illustrated in FIG. 3(j). The pulsesshown in FIG. 3(j) are of a constant amplitude and variable width, thewidth or duration of these pulses being dependent on the phasedifference between the input voltage and current.

The pulse signal shown in FIG. 3(j) is applied through a resistor 44 toa further operational amplifier 46 and a capacitor 48 connected to forman integrator 50.

The command signal referred to above is derived from a potentiometer 52which is set with motor 16 unloaded and, as explained in U.S. Pat. No.4,266,177 referred to above, provides a selected power factor or phaseangle between current and voltage as determined by the greatest powerfactor (smallest motor current-voltage phase difference) at which themotor will operate for the loading range to be encountered. The tap ofpotentiometer 52 is connected to the negative input of amplifier 46through a resistor 54. The positive input is connected to ground througha resistor 56.

The output of integrator 50 is the control signal referred to above andis, as stated, connected to the negative (inverting) input ofoperational amplifier 34.

The circuitry described thus far is similar to that for system describedin U.S. Pat. No. 4,266,177. In accordance with the invention asincorporated in such a system, a further operational amplifier 58 isprovided, the positive (non-inverting) input of which is connected tothe summing point 43 and the negative (inverting) input of which isconnected to receive a positive bias or reference voltage developed by avoltage divider formed by resistors 60 and 62. The output voltage fromamplifier 58 is connected through a resistor 64 to a summing point 66connected to the base of a control transistor 68. The output ofoperational amplifier 34 is connected through a resistor 70 to summingpoint 66 and thus to transistor 68. The emitter of transistor 68 isconnected through a resistor 72 to the gate electrode of traic 18 whilethe collector of transistor 68 is connected to an RC timing circuitformed by a resistor 74 and a capacitor 76.

Considering the operation of the system of FIG. 2, the phase differencesignal at summing point 43 (shown in FIG. 3(j) is conditioned by beingfed to the non-inverting input of amplifier 58. As mentioned above, apositive bias voltage is applied to the inverting input of amplifier 58through the voltage divider formed by resistors 60 and 62. The resultantoutput waveform "k" is shown in FIG. 3(k). This voltage is summed atsumming point 66 with the output of amplifier 34, the latter being afixed level firing pulse as shown in FIG. 3(e) and being derived fromthe ramp output "d" (shown in FIG. 3(d)) and the control signal outputof integrator 50. As will be seen from comparing FIG. 3(d) and FIG.3(e), the firing angle θF is controlled by the intersection of the ramp"d" and the control signal output of integrator 50. Because the emitterof transistor 68 is essentially at ground potential, transistor 68 willbe turned on when the base drive therefor drops negative. Thus, bothinput signals "e" and "k" must simultaneously be negative in order toturn transistor 68 on. In this regard, if both inputs are positive,transistor 68 is off whereas if one is negative and the other positive,the two inputs sum to zero as is indicated by the comparing thewaveforms shown in FIGS. 3(e) and 3(k), and thus transistor 68, is,again, off (no base current will flow with a zero volt base drive).Thus, the signals "k" and "e" are effectively "ANDed" and transistor 68will turn on only when both are negative.

When transistor 68 is turned on, triac 18 is also turned on, with gatecurrent flowing from the ground terminal thereof, indicated at 18a,through gate terminal 18b, resistor 72, transistor 68, and the RCnetwork formed by resistor 74 and capacitor 76, to the negative supply.The amount of current flow, which is typically 50 to 100 milliamps, isdetermined by the value of resistor 72. The length of time this currentflows, which is typically 10 microseconds, is determined by the RC timeconstant. In this regard, a current flow of 100 milliamps for 10microseconds can be readily be supplied by the filter capacitor (notshown) associated with power supply 14. Resistor 74 provides a dischargepath for capacitor 76 during each half cycle.

If the firing angle θF, shown in FIG. 3(e) as varied by the power factorcontroller in response to a varying load, is greater than the referenceangle, θR, shown in FIG. 3(k), the turn on time of triac 18 will becoincident with the firing angle θF. As the load increases on the motor16, the firing angle or point θF will advance in time, i.e., move to theleft in FIG. 3(e), so as to increase the on time of transistor 68, untilthe firing angle θF is equal to or greater than reference angle θR. Thislatter situation is illustrated in FIG. 3(l) and by firing angle θF' andthe dashed line waveform in FIG. 3(e). Under these circumstances, thebase drive for transistor 68 is the signal shown in FIG. 3(m). It willbe seen that transistor 68 will not turn on at the firing angle θF sincethe base drive voltage is zero at that time, and will remain zero untilthe time θR. At the time θR, both signals "l" and "k" are negative andtransistor 68 is turned on to provide firing of triac 16.

Although the invention has been described with respect to a preferredembodiment thereof, it will be understood by those skilled in the artthat variations and modifications can be effected in this exemplaryembodiment without departing from the scope and spirit of the invention.

I claim:
 1. A thyristor control system for an alternating current inputto a load which produces a difference in phase between the load currentand voltage waveforms and comprising a thyristor for controlling thecurrent flow through the load, the improvement comprising pulseproducing means for producing firing pulses for firing said thyristorthe duration of which is short relative to the duration of thealternating current half cycle and is independent of the load, means forderiving a control signal for said thyristor based on the phasedifference between the load current and voltage, and means responsive tosaid control signal for inhibiting production of a firing pulse forfiring said thyristor until a time when no current is flowing from theprevious half cycle of the alternating current input.
 2. A thyristorcontrol system as claimed in claim 1 wherein said pulse producing meanscomprises an electronic switch device and said inhibiting meanscomprises means for controlling the on time of said electronic switchdevice.
 3. A thyristor control system as claimed in claims 1 or 2wherein said inhibiting means including means responsive to said controlsignal for establishing a reference point in time during a half cycle inadvance of which production of a firing pulse for firing said thyristoris inhibited and at which time a firing pulse is produced.
 4. Athyristor control system as claimed in claim 1 wherein said pulseproducing means comprises a transistor and said inhibiting meanscomprises means for ensuring that the base drive for said transistor issuch as to prevent the transistor from being turned on prior to areference point in time related to the phase difference between the loadcurrent and voltage and for providing a firing pulse for turning on saidtransistor at said reference point in time.
 5. A thyristor controlsystem as claimed in claim 4 wherein the base of said transistor isconnected to receive a first signal from said inhibiting means and asecond signal from a control circuit for controlling firing of saidthyristor, said thyristor being connected such that said first andsecond signals must both be of the same polarity before said transistoris turned on.
 6. A thyristor control system as claimed in claim 1wherein said thyristor control system comprises a power factorcontroller and said means for deriving a control signal based on thephase difference between the load current and voltage is responsive tophase difference signals existing in the power factor controller.
 7. Athyristor control system as claimed in claim 1 wherein said loadcomprises an induction motor and said thyristor control system comprisesa power factor controller for controlling the power supplied to saidmotor in accordance with the phase difference between motor voltage andcurrent waveforms, and said control signal deriving means derives saidcontrol signal from signals produced by said power factor controller. 8.A thyristor control system as claimed in claim 7 wherein said powerfactor controller produces first and second signals proportional thesquare of to the load voltage and the inverse thereof, and third andfourth signals proportional to the square of the load current and theinverse thereof, and includes summing means for selectively summing saidsignals to produce a first square wave having a pulse durationproportional to the phase shift for a positive half cycle of the inputcurrent and a second square wave having a pulse duration proportional tothe phase shift for a negative half cycle of the input current, saidcontrol signal being derived from said square waves.
 9. In a thyristorcontrol system for an alternating current input to a load which producesa difference in phase between the load current and voltage waveforms,said system comprising means for sensing the load current and voltageand for producing a phase difference output signal proportional to thephase difference between the load current and load voltage; means forgenerating a preselected power factor command signal; means responsiveto said phase difference output signal and said power factor commandsignal for producing a first control signal; ramp generator means forsensing said alternating current input and for generating a voltage rampin timed relation to said alternating current input; means responsive tosaid voltage ramp and said first control signal for producing a secondcontrol signal; a thyristor for controlling the current flow through theload; and control means responsive to said second control signal forcontrolling switching of said thyristor, the improvement wherein afurther means responsive to said phase difference output signal isprovided for generating a third control signal and wherein said controlmeans includes means responsive to said second control signal and saidthird control signal for generating firing pulses for firing saidthyristor and for inhibiting generating of a firing pulse for firingsaid thyristor until a time when no current is flowing from the previoushalf cycle of the alternating current input.